workspacesView: Center extra workspace

workspacesView: Center extra workspace

At the moment views on non-primary monitors take up the entire work area,
so simply allocating the available size works. However we'll soon shrink
the views a bit to match the visuals on the primary monitor. As workspaces
keep the ratio, reducing their height will also reduce the width; override
the default allocate() to keep the extra workspace horizontally centered.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1735>